It is well known that harvested nuts should generally be processed and hulled within a relatively short period of time after harvest. Pistachio nuts are particularly problematic in that if the hulls of pistachio nuts are not promptly removed from the shells after harvest, the light color of the shell may become blemished, giving the pistachio nut a less attractive appearance, and reducing its market value. In addition, there is a bond between the hulls and shells of pistachio nuts, making it more difficult to remove the hulls, especially if the pistachio nuts are not fully ripened.
Modern growing practices have resulted in large volumes of nuts being harvested at or around the same time, requiring large scale hulling and processing to be accomplished during a relatively short window of time. Accordingly, numerous mechanical devices and machines have been developed for large scale processing and hulling of nuts. It is desirable for such nut harvesting machines to avoid nut breakage as much as possible, to provide efficient processing (hulling/peeling) of nuts, to provide a high percentage of processed nuts, to provide a good ratio of capacity relative to machine foot print, and to provide consistent performance regardless of nut sizes and/or nut varieties. Unfortunately, current pistachio dry hulling machines are unable to meet one or more of these standards, resulting in losses from unprocessed nuts, broken or damaged nuts, and/or unacceptably lengthy processing times.
Many existing dry nut hulling machines include a rotatable drum having a pattern of protrusions or bolts on the surface thereof, a hopper that feeds the nuts into contact with the drum, the hopper including a front plate and a back plate for guiding the nuts. See, Shamsi, M., et al., Performance evaluation of a bolt type pistachio hulling machine, Journal of Agricultural Technology, 2011 Vol. 7(1), 57-62 (2011). See FIG. 1.
In such systems, the hulls of pistachio nuts are removed from the shells as a result of the impingement of the protrusions on the surface of the drum against the hulls. In such machines, a back plate (“Plate 2” of Shamsi) is located in the upper half circumference of the drum in one quadrant, and a front plate (“Plate 1” of Shamsi) is located in an adjacent upper quadrant upstream of the direction of drum rotation. Nuts are fed into the hopper area between the front and back plate. As the drum is rotated, the nuts are urged toward the back plate, which is positioned above the protrusions on the drum. The nuts are impinged by the protrusions on the drum, causing the hulls to be cut, sliced and/or sheared away from the nut shells. The hull slices or peels (“Skin discharge” of Shamsi) then pass through a gap between the back plate and the drum. This gap is too small for the hulled nuts to exit, so they are forced upward against the back plate until they fall back toward the drum, where they move in a rotational direction that is counter to the rotation of the drum. As drum rotates in one direction, it causes a counter rotation of nuts that are on the drum and trapped by the front and back plate to rotate collectively in the opposite direction of the drum rotation in a semi-fluid fashion. Eventually the hulled nuts (“Hulled discharge” of Shamsi) travel toward the front plate, and exit through a different gap between the front plate and the drum. The front plate gap therefore acts as a “sizer” for the nuts being processed, allowing peeled nuts to pass through it but not unpeeled nuts. The opening size of this gap determines the maximum diameter of nuts that may exit from the machine. Unfortunately, in many cases (particularly when a wider gap is used for larger nuts), it is possible for smaller un-hulled nuts to also exit through this gap between the front plate and the drum, resulting in fewer nuts actually being processed by these machines, thereby lowering the percentage of usable, hulled nuts.
As noted in Shamsi et al, the tested pistachio hulling machines only processed an average of approximately 73.8% of nuts, with an average of approximately 25.5% of nuts remaining unprocessed. Thus, approximately one quarter of all nuts entering such machines were not processed. It is therefore desirable to improve the percentage of nuts actually processed by pistachio and other nut hulling machines without increasing processing time or nut breakage.
One attempt at improving nut hulling is found in U.S. Patent Publication No. 2015/0223514 (Nakhei-Nejad—'514). The '514 application discloses a hulling plate that may be adjusted to establish a gap between the hulling plate and the drum through which removed hulls to exit from the machine, and a separator plate that may also be adjusted to establish a different gap between the separator plate and the drum through which hulled or peeled nuts may exit from the machine. The sizes of these gaps are not disclosed, although in one embodiment the separator gap is angled such that it is wider at one end, and a helical pattern of protrusions is provided on the drum which urges the nuts from one end of the drum to the other, where the wider gap is located. Unfortunately, the invention of the '514 application does not appreciably improve the percentage of nuts actually hulled, and the gradually increasing separator gap will allow many unprocessed nuts through. In particular, the separator gap of the '514 application plate still remains a sizer as it only allows nuts to exit based on their size at any given point on the drum. So, if a particular size nut that is not peeled by the time it reaches the gap having a size bigger than the nut diameter, that unpeeled nut will fall through unprocessed; and a nut that is peeled early in the process cannot exit until it reaches a part of the opening that is wider than its diameter. Also, the front and back plates of the '514 application are located in different quadrants.
Another attempt at improving nut hulling is found in U.S. Patent Publication No. 2015/0282518 (Tjerrild—'518). The '518 application discloses adjusting the position of the back plate in a range of between 10° and 80° along the circumference of the drum relative to horizontal, and tilting the back plate anywhere from positive 10° to negative 15°. The back plate has a lower cutting edge that is adjustably positioned approximately 1 mm above the drum protrusions. The '581 application is silent with respect to the position of the front plate relative to the drum protrusions. In the '518 application, the back plate is located in one quadrant (18), and the front plate is located in another quadrant (16). The '518 application discloses the use of a chevron or herring-bone pattern of protrusions which contact the nuts at an oblique angle and urge the nuts toward the center of the drum. Unfortunately, the invention of the '518 application does not appreciably improve the percentage of nuts actually hulled, does not compensate for different sized nuts, may result in undesirable accumulation of un-hulled nuts on the back plate, and may cause substantial nut breakage at the back plate. Lateral removal of the drum also requires removal of one or both of the plates first.
Accordingly, there remains a need in the nut industry, and particularly in the pistachio industry, for automated hulling machines which provide a low percentage of nut breakage (less than 1%), high peeling efficiency, a high percentage of processed nuts (greater than the average of 73.8%), a good ratio of capacity relative to machine foot print, and consistent performance regardless of nut sizes and/or nut varieties. The apparatus and methods of the present invention address all of these needs.